Device for acquiring a three-dimensional shape by optoelectronic process

ABSTRACT

The invention concerns a device for acquiring the three-dimensional shape of an object ( 10 ) by opto-electronic process, comprising a chromatic system ( 18 ) for illuminating the object ( 10 ) and for picking up the light reflected or backscattered by the object ( 10 ), and a reflecting mirror ( 26 ) placed on the optical axis between the optical system ( 18 ) and an illumination slot ( 16 ) for deflecting the light reflected by the object towards a spectral analysis means ( 30 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The application claims priority of international application00/04954 filed Apr. 14, 2000.

FIELD OF THE INVENTION

[0002] The invention relates to an apparatus for opto-electronicallyacquiring a three-dimensional shape, of the type described ininternational application

BACKGROUND OF THE INVENTION

[0003] International application WO99/64816 describes an apparatus foracquiring shapes comprising lighting means, the lighting meanscomprising a luminous polychromatic source and a chromatic lens, thelighting means coupled to an optical system for magnification, means forsensing light reflected or backscattered by an illuminated object, andspectral analysis means for analyzing sensed light, coupled to dataprocessing means. The spectral analysis means are on the optical axis ofthe light sensing means, while the lighting means are offset angularlyand illuminate the object by means of a semi-transparent blade placed onthe optical axis of the light sensing means.

[0004] This known apparatus performs to a satisfying degree, with ameasurement depth that is relatively high and notably superior to otherapparatus in the state of the art.

SUMMARY OF THE INVENTION

[0005] Accordingly, an object of the present invention is to perfect theapparatus described above and improve its performance.

[0006] According to a first broad aspect of the present invention, thereis provided an apparatus for opto-electronically acquiring threedimensional shapes, the apparatus comprising a luminous source with acontinuous spectrum, an illumination slot provided on the optical axisin front of the source, an optical image forming system, an opticallight sensing system for sensing light reflected by the object, spectralanalysis means for analyzing the spectrum of the light sensed by thelight sensing means, and data processing means coupled to the spectralanalysis means, wherein the image formation system and the image sensingsystem are comprised within one optical chromatic system and whereinlight reflected by the object and being output by the optical chromaticsystem is reflected off a mirror placed on the optical axis between theillumination slot and the optical system and is deflected towards ananalysis slot placed on the optical axis of the spectral analysis means.

[0007] The apparatus, according to the present invention, providesseveral advantages with respect to other known techniques. the depth ofthe field and the measurement depth are increased. The apparatus is lesssensitive to parasitic wavelengths and the measurements are moreprecise.

[0008] Alternatively, the mirror is a mask that intersects the lightrays output from the illumination slot on the optical axis towards theobject. This mask avoids illuminating a point on the surface of theobject with a set of rays of different wavelengths and facilitateslocating the point on the optical axis by illuminating with only onewavelength.

[0009] Preferably, the optical chromatic system is afocal and an opticalmagnification system, preferably afocal, is mounted on the optical axisbetween the optical chromatic system and the illuminated object.

[0010] In order to obtain a telecentric path of rays and to reduceshadow zones on the illuminated object, a mask comprising a circularhole can be placed on the optical axis to allow reflected rays to passthrough, the mask being placed in the centre of the optical chromaticsystem and comprising at least two lateral orifices for the passage ofillumination light for the object.

[0011] In order to increase the signal to noise ratio and thereforeincrease the precision of the measurement (or the gain of the signalwhen it is weak), the spectral analysis means can comprise a matrix ofcameras of type CCD or analog, wherein the outputs of the cameras arecoupled to means for analog to digital conversion via low pass filtersthat eliminate noise due to high frequencies on analog signals at theoutput of the sensing means.

[0012] According to another alternative embodiment, the apparatus alsocomprises optical image rotation means placed on the optical axis in anarea intersected by the illuminating light of the object and by thelight reflected by the object, the image rotation means comprising, forexample, a DOVE (Distributed Object Visualization Environment) prismthat can be placed in the optical chromatic system.

[0013] By rotating the prism around the optical axis, we can rotatearound the optical axis the measurement profile on the illuminatedobject. This allows a rotating sweep of the illuminated surface of theobject without relative displacement between the acquiring means and theilluminated object in order to acquire the three-dimensional shape in acylindrical volume of measurement that is defined by the rotation of themeasurement profile around the optical axis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription and accompanying drawings wherein:

[0015]FIG. 1 is a schematic of the apparatus according to the invention;

[0016]FIG. 2 is a simplified partial view illustrating the operation ofthe apparatus;

[0017]FIG. 3 is a frontal view of a mask used for obtaining atelecentric path of rays;

[0018]FIG. 4 is a frontal view of an alternative embodiment for the maskof FIG. 3;

[0019]FIG. 5 is a schematic showing the optical image rotation meanswithin the apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] In FIG. 1, reference numeral 10 designates an object for which wewant to acquire a three dimensional shape, the object being on theoptical axis 12 of an apparatus according to the invention, theapparatus comprising an illuminating source 14 with a continuousspectrum, connected by, for example, optical fibers to the focal pointof an optical system 15 which is a condenser that focalizes lightemitted by the source 14 through an illumination slot 16 placed on theoptical axis 12.

[0021] The slot 16 is followed by an optical chromatic system 18 whichin this case is an afocal system, comprising two identical chromaticlenses 20, the slot 16 being at the focal point of the first lens 20.

[0022] The optical chromatic system is followed by an optical magnifyingsystem 22, preferably afocal.

[0023] The illuminated object 10 is placed approximately at the focalpoint of the last lens 24 of the optical magnifying system.

[0024] The optical chromatic system 18 comprises the light sensingsystem to sense light reflected or backscattered by the surface of theobject 10, and a reflecting mirror 26 is placed on the optical axisbetween the illuminating slot 16 and the first lens 20 of the chromaticsystem 18 in order to deflect, preferably perpendicularly to the opticalaxis 12, the light sensed, in the direction of the analysis slot 28placed on the optical axis of the spectral analysis means 30 that iscoupled to data processing means 32.

[0025] The spectral analysis means 30 are, for example, such as thosedescribed in international patent application WO99/64816, herebyincorporated by reference.

[0026] The apparatus described according to the present inventionprovides the advantage that, with respect to the apparatus described inthe document cited above and incorporated by reference, it does not usesemi-transparent blades placed on the optical axis to illuminate theobject and sense the luminous flux reflected or backscattered by theilluminated object. The use of a semi-transparent blade necessarilyimplies that a major portion of the luminous flux reflected orbackscattered by the illuminated object is lost. In the apparatus asshown in FIG. 1, the gain of the luminous flux reflected orbackscattered by the object 10 is within a range of 2 to 4,approximately, with respect to the embodiments described in previousdocuments cited above.

[0027] Another advantage of the apparatus according to the inventionwill now be described in reference to FIG. 2.

[0028] In this figure, in order to simplify it, the optical chromaticsystem 18 is represented by a single lens and the mirror 26 isrepresented by a mask that is placed on the optical axis 12 and thatintersects the central polychromatic light rays exiting the illuminationslot 16.

[0029] Since the optical system 18 is a chromatic system with a focallength that varies continuously with wavelength, the differentwavelengths exiting the illumination slot 16 are focalized at differentpoints on the optical axis 12. For example, a luminous ray R1 havingwavelength λ1 is focalized at point P1, and a luminous ray R2 havingwavelength λ2 greater than λ1 is focalized on the axis 12 at a point P2which is further from the chromatic system 18 than point P1.

[0030] If the wavelengths of rays R1 and R2 correspond to theextremities of the wavelength illumination band, than the distance P1-P2on the optical axis 12 represents the measurement depth.

[0031] Since the central luminous rays exiting the illumination slot 16are intersected by the mask 26, we can say that point P1 on the axis 12is illuminated uniquely or almost uniquely by a light of wavelength λ1,point P2 will be illuminated by a light of wavelength λ2, and a point P1in between P1 and P2 will be illuminated by a light of wavelength λ1 ina range between λ1 and λ2.

[0032] Since the optical chromatic system 18 comprises the opticalsensing system that senses light reflected or backscattered by theobject, the light rays of wavelength Al reflected or backscattered bythe point P1 are focalized on the optical axis 12 at the illuminationslot 16, the light rays of wavelength λ2 reflected or backscattered bythe point P2 are also focalized on the illumination slot 16, and thelight rays of wavelength λ1 reflected by the intermediate point P1 arefocalized on the illumination slot 16.

[0033] This shows that a clear image can be formed from any point on theaxis between points P1 and P2 by the chromatic system 18 on the spectralanalysis means, without adjustment to the apparatus.

[0034] This also shows that if a parasitic light ray having a wavelengthdifferent from λ1 is reflected by point P1, this parasitic light raywill not be focalized at the illumination slot 16 and a clear image ofpoint P1 at the wavelength(s) of the parasitic light ray will not beformed on the spectral analysis means.

[0035] The optical chromatic means 18 therefore allow a spectralfiltering of the sensed light to occur, while ensuring the clearness ofthe image formed on the spectral analysis means on the entire range ofmeasurement depth useful along the optical axis 12, and the measurementsare more precise.

[0036] The analysis slot provides a spectral filtering and a spatialfiltering of the sensed light.

[0037] To improve the performance of the apparatus, a telecentricanalysis channel that avoids or reduces the shadow zones on theilluminated object 10 is provided via a mask 34 shown in FIG. 3 andplaced at the center of the optical chromatic system 18, the mask 34comprising a central zone 36 of intersection of the light rays, providedby a circular axial hole 38 that lets reflected light through, that is,light reflected or backscattered by the object 10.

[0038] The illumination light exiting the slot 16 passes on each side ofthe central section, as shown by reference numeral 40.

[0039] Alternatively, and as shown in FIG. 4, the sections 40 for theillumination light to pass through can be reduced to circular orificesformed within a disc 34 comprising a central hole 38 for reflected lightto pass through.

[0040] With mask 34 of FIG. 4, a vertical telecentric illumination pathis provided.

[0041] As already stated in prior art reference WO99/64816, herebyincorporated by reference, a profile image of the illuminated surface ofthe object 10 can be formed on spectral analysis means 30, whichcomprise a matrix of cameras, such as CCD or analog cameras. Arotational sweep of the illuminated surface of the object 10 can be doneusing an optical image rotation system, such as a DOVE prism placed inthe apparatus according to the invention on the optical axis 12, in azone that is intersected by the illumination light and by the sensedlight. For example and as shown in FIG. 5, the DOVE prism 42 can beplaced between two lenses 20 of the chromatic system 18. The rotation ofthe DOVE prism 42 around the optical axis 12 produces the result ofrotating around the axis the measurement profile on the illuminatedsurface of the object 12, the rotation of the profile being twice asgood as that of the prism. We can then obtain, without any otherdisplacements of the apparatus according to the invention or of theobject 10, a three dimensional shape of the surface of the object 10illuminated by a luminous cylindrical axis 12 which has a diameter equalto the length of the measurement profile.

[0042] To increase the signal to noise ratio of the signals exiting theCCD sensors of the camera matrix, a low pass filter is used to connectthe sensor outputs to means for analog to digital conversion thatconnect the spectral analysis means 30 to signal processing means 32.Typically, the cut-off frequency of the low pass filter is approximately1 MHz.

[0043] In the embodiment of the apparatus shown in FIG. 1, the lenses 20of the chromatic system 18 have a focal length of 60 mm, the lens 24 ofthe optical magnifying system has a focal length of 150 mm, thetransversal magnifying ratio of the system 22 is 3, and the measurementdepth on the optical axis 12 is approximately 40 mm. The image formed onthe analysis means is clear without any adjustments on the entiremeasurement depth (in prior art techniques, the range of clearness ofthe image is approximately ±4 mm from a central position according to asetting). Furthermore, the spectral width of the signal is approximatelytwo times smaller in the apparatus according to the invention than inthe prior art techniques, and the precision is uniform on the entiremeasurement depth (0.01%) instead of having a bell curve-type variationwith a progressive degradation when moving away from the centralposition.

1. An apparatus for opto-electronically acquiring three dimensionalshapes of an object, the apparatus comprising: a continuous spectrumlight source (14); an illumination slot (16) on an optical axis in frontof the source (14); an optical chromatic image forming system (18) forforming an image on the object (10) and sensing light reflected by theobject (10); spectral analysis means (30) for the reflected light; anddata processing means (32) coupled to the spectral analysis means (30);characterized in that it comprises a mirror (26) placed on the opticalaxis (12) between the illumination slot (16) and the optical system (18)to intercept the light rays output from the illumination slot (16) onthe optical axis (12) in the direction of the object (10) and to alsointercept light reflected from the object (10) and output from theoptical system (18) and deviate it, for example perpendicularly to theoptical axis (12), towards an analysis slot (28) placed on the opticalaxis of the spectral analysis means (30).
 2. An apparatus as claimed inclaim 1, characterized in that the optical chromatic system (18) isafocal.
 3. An apparatus as claimed in claims 1 or 2, characterized inthat the optical chromatic system (18) is formed by two identicalchromatic lenses (20).
 4. An apparatus as claimed in one of thepreceding claims, characterized in that a magnifying optical system (22)is mounted on the optical axis (12) in between the optical chromaticsystem (18) and the object (10).
 5. An apparatus as claimed in claim 4,characterized in that the magnifying optical system (22) is afocal. 6.An apparatus as claimed in one of the preceding claims, characterized inthat a mask (34) comprising a circular axial hole (38) for passingreflected light from the object (10) and two orifices (40) for passinglight illuminating the object (10) is placed at a centre of the opticalchromatic system (18).
 7. An apparatus as claimed in one of thepreceding claims, characterized in that the spectral analysis means (30)comprises a matrix of sensors of type CCD or analog having outputsconnected to means for analog to digital conversion via a low-passfilter.
 8. An apparatus as claimed in one of the preceding claims,characterized in that it comprises optical image rotation means (42)placed on the optical axis (12) in a zone traversed by the illuminationlight for the object and by the light reflected by the object.
 9. Anapparatus as claimed in claim 8, characterized in that the optical imagerotation means are a DOVE prism (42) placed for example in the opticalchromatic system (18) cited above.